(1) Field of the Invention
The present invention relates to towed array signal processing and more particularly to a means for extending the physical aperture of an array of hydrophones by exploiting the movement of this array during an observation period such that the effective aperture is equivalent to that of a longer, fully populated array having length equal to the distance travelled by the moving array during this observation period, such means using a novel, overlap correlator based, synthetic aperture signal processing technique.
(2) Description of the Prior Art
A continuing goal of sonar system designs in general is to increase the detection range. Towed array type sonar systems were introduced because they permitted listening at lower frequencies which in turn provided greater detection ranges. Since the low frequency regime included substantial traffic noise however, very high bearing resolution was required in order to permit angular separation of closely spaced sources, especially while operating in an environment having very high traffic density. This requirement for providing higher bearing resolution at low frequencies lead to development and use of longer towed hydrophone arrays. Use of longer arrays however had serious, undesirable technical and operational implications. In order to avoid physical extension of towed arrays, many attempts were made to increase the effective length of a given array by synthesizing additional hydrophones. Such attempts included application of aircraft and satellite active radar system synthetic aperture techniques. The results of these attempted applications to sonar systems were not successful however due to the presence of disturbed motion of the towed array caused by currents and also due to lower coherence of acoustic signals in seawater as compared to the coherence of electromagnetic waves in the air.
More recent experimental studies, (WILLIAMS, R. E., Creating an Acoustic Synthetic Aperture in the Ocean, Journal of the Acoustical Society of America, 1976, Vol.60, pp 60-73; BROEK, H. W., Temporal and Spatial Fluctuations in Single-path Underwater Acoustic Wave Fronts, Transmission from the First Convergence Zone at 43-nmi Range, Journal of the Acoustical Society of America, 1982, Vol.72, pp 1527-1532; and KOENIGS, P. D. et al, A Further Study of the Space and Time Stability of a Narrowband Acoustic Signal in the Ocean, Short Range Results, Presented at the 102H nd Meeting of the Journal of the Acoustical Society of America, Dec. 1, 1981, Miami Beach, Fla.) NUSC Technical Document 6605,21 Dec. 1981, however, indicated that the space and time coherence of the acoustic signal in the sea appears to be sufficient to synthetically extend the physical aperture of a Moving Towed Array (MTA). Unfortunately, the approach used in what is now the "standard" synthetic aperture technique requires a highly accurate a prior knowledge of the source frequency. In addition, even when the source frequency is known, processing still suffers degradation due to random array motion and anomalies in the propagation medium. What is needed is a means for extending the effective aperture of an array beyond its physical aperture without having to contend with the above limitations.